The present invention relates to a semiconductor light emitting device which has a double hetero structure portion in which an active layer is sandwiched by semiconductor layers having a larger band gap than that of the active layer such that a light emitting layer is formed, such as a light emitting diode (LED), a laser diode (LD) having stripe groove and a vertical cavity surface emitting laser (VCSEL) which emits laser light from a surface of the laminated semiconductor layers, and a manufacturing method thereof. In particular, the present invention relates to a semiconductor light emitting device in which a radiating efficiency from side wall portions of the double hetero structure portion can be improved, light leakage can be reduced so as to enable efficient light emission in a narrow area and an efficiency of light outputted with respect to light inputted can be improved, and a manufacturing method thereof.
A conventional surface emitting type semiconductor laser manufactured by a simple process has a structure shown by cross-sectional explanatory views in FIGS. 5A and 5B. In FIGS. 5A and 5B, a lower multi-layer reflection film 62 which is formed of a laminated structure referred to as DBR (Distributed Brag Reflector) is formaed on a semiconductor substrate 61 formed of, e.g., GaAs. On the lower multi-layer reflection film 62, a lower spacer layer 63, an active layer 64 and an upper spacer layer 65 are successively grown. On the upper spacer layer 65, an upper multi-layer reflection film 67 formed of DBR is formed. Then, as shown in FIG. 5A, an insulating area 68 is provided on an outer periphery side of a current injection area by implanting ions such as protons. Alternatively, the outer periphery side may be removed by etching as shown in FIG. 5B. Unillustrated upper and lower electrodes are respectively provided on the surface of the upper multi-layer reflection film 67 and on a rear surface of the semiconductor substrate 61 such that laser beam light can exit from a part of the upper surface.
In accordance with the above-described structure, current is injected into a narrow area serving as the current injection area, and then light with high intensity is emitted. Further, the upper and the lower multi-layer reflection films 62 and 67 resonate as a reflecting surface of cavity resonator. Thus, laser resonance occurs and a part of resonated light exits as laser beam light from a small exit opening (not shown) formed at the upper electrode through the upper multi-layer reflection film 67 which is formed so as to have a smaller reflectance than the lower multi-layer reflection film 62.
As described above, in a conventional surface emitting type semiconductor laser, optical confinement in a vertical direction is accomplished by the upper and the lower multi-layer reflection films and light emission in a horizontal direction does not occur by insulation or removal such that efficient light emission can be accomplished by injecting the current only in the light emitting area. However, since the current centralizes only in the narrow central portion and only the area emits light, an increase in temperature in the area is significant. In the structure shown in FIG. 5A, as the semiconductor layer serving as an insulating area is formed on the periphery of the light emitting area, heat capacity is relatively large. However, the semiconductor layer used for a light emitting device does not have so large heat conductivity and heat generated in the light emitting area cannot be sufficiently diffused. Thus, the semiconductor layer is easily deteriorated and a luminous efficiency is also easily decreased. Especially in the structure that the periphery of the current injection area is removed by etching as shown in FIG. 5B, heat generated at a time of light emitting cannot be diffused efficiently. For this reason, there arise problems in that the luminous efficiency is decreased and the semiconductor layer is partially damaged such that a lifetime of the laser is reduced.
The current is centralized on the narrow area to carry out efficient emission. However, in the surrounding non light emitting area, the semiconductor layer has the same composition as that of the light emitting area into which the current is injected. The non light emitting area is formed so as to have larger electric resistance due to crystal defects being generated by ion implantation. The light emitted at the current injection area easily travels toward the insulting area, so that light cannot be confined only in the current injection area. For the light which has traveled into the insulating area, according to a difference of refractive index between the insulating area and an air layer, around 30% of reflectance can be obtained at a side surface of the insulating area. At the same time, however, an amount of light leaked is significantly large. In this problem about optical confinement, even the structure shown in FIG. 5B that the outer periphery of the current injection area is removed by etching has an effect of optical confinement of around 30% with respect to total reflection on a basis of a difference of refractive index derived from direct contact with air. Accordingly, there arise problems in that a resonance efficiency cannot be sufficiently increased and thus a threshold is increased.
Such problems are not limited to the surface emitting type semiconductor laser. An LED and an LD having stripe groove have problems in that light leaks from a chip side surface of the LED or the LD (from a side surface which is perpendicular to a laser light exiting surface in a case of LD) and thus is wasted.
The present invention was devised in order to solve the above-described problems, and an object of the present invention is to provide a semiconductor light emitting device that radiating characteristic of the periphery of a light emitting area of the semiconductor light emitting device which emits light by injecting current into a part of area can be improved, an optical confinement efficiency of the periphery of the light emitting area can be improved, and thus an efficiency of light outputted with respect to a certain input can be improved, a reliability can be improved by reducing damages of semiconductor layer, and a manufacturing method thereof.
Another object of the present invention is to provide a structure for enabling improvement of heat diffusion of surface emitting type semiconductor laser in which a vertical cavity is formed by making an area of double hetero structure portion into which current is injected smaller than an area of substrate and for enabling improvement of an efficiency of light outputted, and a manufacturing method of the surface emitting type semiconductor laser.
In accordance with the present invention, a semiconductor light emitting device includes: a semiconductor substrate; and a double hetero structure portion formed on the semiconductor substrate and in which an active layer having small band gap is sandwiched between semiconductor layers having larger band gap than that of the active layer, wherein a heat radiating film which has light reflecting property and superior heat conductivity than that of the double hetero structure portion is formed at least a part of side walls of the double hetero structure portion. The side wall refers to as a surface which extends in a direction vertical to a surface of the semiconductor layers are laminated. In a case of the semiconductor element, the semiconductor layers may be laminated so as to obtain an LED, a stripe type LD or a surface emitting LD.
Because of this structure, light which is emitted from the active layer and travels toward the side walls is not emitted from the side walls and confined within the light emitting area. Further, the light contributes to oscillation within a resonator in a case of the semiconductor laser, or to emit light in a predetermined direction in a case of the LED. In both of the case of the semiconductor laser and the case of the LED, an efficiency of light outputted with respect to light inputted is improved, and heat generated due to centralization of current and light emitting can be rapidly diffused through a heat radiating film, so that troubles caused by heat generation can be prevented. Consequently, the present invention contributes significantly to light emitting efficiency and reliability.
The heat radiating film is directly applied on the periphery of a light emitting area of the double hetero structure portion. Thus, radiation can occur rapidly and the emitted light can be confined without being wasted because of the light reflecting property.
The heat radiating film is formed as a composite film made of an insulating film which is applied to the side wall of the double hetero structure portion and a metallic film which is applied to outer side of the insulating film. Since the metallic film which has superior radiation and light reflection to the insulating film can be used, an increase in temperature can be prevented efficiently by the metallic film having especially excellent heat conductivity. When a metal oxide film is used as the insulating film, a metallic film can be formed at the side walls by a plating method, and then a light reflection film which has excellent light reflection and heat diffusion can be formed by the plating method and an oxidization treatment. At least one of Cr, Ni, Cu, Pt, Ag and Al can be used for the metallic film.
When the heat radiating film is made of materials including at least one of diamond, diamond-like carbon and alumina, because these materials have excellent heat conductivity in spite of being the insulating film, heat generated in the light emitting area can be rapidly diffused while preventing short of opposite sides of the active layer of the double hetero structure portion.
A surface emitting type semiconductor laser diode of the present invention includes: a semiconductor substrate; a lower multi-layer reflection film formed on the semiconductor substrate; a double hetero structure portion which is formed on the lower multi-layer reflection film and in which an active layer with small band gap is sandwiched between semiconductor layers with larger band gap than that of the active layer, and an upper multi-layer reflection film formed on the double hetero structure portion, wherein the upper multi-layer reflection film and the double hetero structure portion on the periphery of a certain area serving as a current injection area of the double hetero structure portion are removed by etching, a heat radiating film which has light reflecting property and superior heat conductivity than that of the double hetero structure portion is formed at a side wall exposed by etching. The present invention is configured so as to be a vertical cavity type laser which emits light exits from an upper surface side of the upper multi-layer reflection film.
A method of manufacturing a semiconductor light emitting device includes the steps of: laminating semiconductor layers on a semiconductor substrate so as to form a double hetero structure portion in which an active layer with small band gap is sandwiched between semiconductor layers with larger band gap than that of the active layer; etching a part of the laminated semiconductor layers so as to expose a side wall of the double hetero structure portion; and applying a heat radiating film which has light reflecting property and superior heat conductivity than that of the double hetero structure portion to at least a part of the exposed side walls of the double hetero structure portion.
The heat radiating film may be formed such that a metallic film is formed by a plating method, and then the resultant metallic film is oxidized. Alternatively, materials for the heat radiating film may be deposited on the side walls. In a case of oxidizing the metallic film, it is preferable to additionally form a metallic film such that the radiating property can be improved.
A method of manufacturing a surface light emitting type semiconductor laser of the present invention includes the steps of: (a) growing, on a first conductive type semiconductor substrate, a lower multi-layer reflection film which is formed by laminating first conductive type semiconductor layers; (b) growing, on the lower multi-layer reflection film, a double hetero structure portion formed of a first conductive type lower spacer layer, an active layer made of semiconductor having smaller band gap than that of the spacer layer and a second conductive type upper spacer layer made of semiconductor having larger band gap than that of the active layer; (c) growing, on the double hetero structure portion, an upper multi-layer reflection film formed by laminating the second conductive type semiconductor layers; (d) removing the upper multi-layer reflection film and the double hetero structure portion on the periphery of the current injection area by etching; (e) exposing only side walls exposed by the etching and applying a metallic film on the exposed surfaces by a plating method; and (f) forming the applied metallic film into a metal oxide film by an oxidation treatment.
In the above method, instead of steps (e) to (f), an insulating film or an insulating film and a metallic film may be directly applied by a CVD method, a sputtering method, a metallic film forming method using particulates, or a vacuum evaporation method.